You have to unlock them by playing them in Challenge Mode.
It's in Japanese, but here's a decent chart that tells you what point values to go for and which EX charts they will let you play. ＣＳポップン６ - チャレンジポイントリスト
The whole process can be kind of grueling in Pop'n 6, it's way harder to get all of the EXs unlocked than in Pop'n 7 or 8.
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i am vengeans. i am teh nite. i, bortman.
i should probably point out somewhere here that i'm dewgy shouldn't i
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